1. Field of the Invention
This invention relates to a system and corresponding method for generating and displaying a representation of a volume of a patient""s body using ultrasonic imaging.
2. Description of the Related Art
In recent years, more and more effort has been directed towards developing and improving the ability of an ultrasonic imaging system to generate a displayable representation of three-dimensional regions of a patient""s body. Unlike the already widely available 2-D imaging systems, which scan and image planes within the body, usually at constant depths; 3-D imaging systems compile imaging information for an entire 3-D region at a time.
Of course, existing display screens are 2-D, so that some 2-D subset (projection) even of 3-D data must be selected for display to the user. There are many different methods for creating a 2-D projection for display.
Three-dimensional imaging systems will, in many cases, allow visualization of body structures in a region of interest that cannot be imaged easilyxe2x80x94or at allxe2x80x94using 2-D scanning. For example, assume that a blood vessel extends perpendicular to the scan plane. In 2-D scanning, this blood vessel will appear, at best, as a small ring. With 3-D scanning, as long as the 2-D projection is chosen properly, then the length of the blood vessel will be visible in the display. Another advantage of 3-D imaging is that it allows a given scanned volume to be examined (displayed) from different points of view.
Several different methods are known for selecting a 2-D subset of 3-D imaging data to be used for display. In other words, there are several different projection methods. According to one method, the system simply displays the image data that lies on a plane of intersection through the scanned volume. In other words, the system chooses a planar xe2x80x9cslicexe2x80x9d of the imaged volume. This can be done using traditional geometric algorithms.
One of the currently most popular projection techniques in 3-D ultrasound imaging is known as the Maximum Intensity Projection (MIP). According to this technique, a point of view is selected, which is usually the center of the piezoelectric array in the ultrasound probe. A projection plane is then selected, which is usually perpendicular to the centered normal of the array and is located between the array and the scanned volume. The plane is divided into an x-y grid of picture elements (xe2x80x9cpixelsxe2x80x9d). A vector is then extended from the point of view through each pixel on the projection plane and into and through the 3-D pattern of imaged volume elements (xe2x80x9cvoxelsxe2x80x9d) that make up the representation of the scanned volume. Each vector will pass through many voxels, each of which, in conventional B-mode imaging, is represented by a brightness value. In MIP imaging, the system determines the maximum brightness value lying on each vector, and then assigns that value to the pixel on the projection plane that the vector passes through. Of course, all of these processing steps are carried out in software, using known mathematical algorithms for computational geometry.
One of the advantages of MIP is that it does not require the user to choose the projection plane, that is, the displayed xe2x80x9cslicexe2x80x9d of the imaged volume. One problem that existing 3-D imaging systems face, however, as does any other type of signal-processing system, is noise.
Whenever an object is scanned by some form of radiation, structures within the object that are too small (more precisely, that are smaller than the wavelength of the scanning signal) to be resolved may still disperse, reflect, or otherwise interfere with the signal that is returned to the scanning device. When the imaging system then creates an image based on the returned scan signal, this interference, which is noise, often makes the image less clear.
In medical ultrasonic imaging, the ultrasonic beam transmitted into the body is scattered by the microstructure of the tissue. This interference effect is known as xe2x80x9cspeckle.xe2x80x9d Speckle causes the image to appear granular, which in turn obscures smaller structures and masks the presence of low-contrast lesions. The problem is analogous to xe2x80x9csnowxe2x80x9d on a television screen, which reduces the xe2x80x9csharpnessxe2x80x9d of the TV image.
Although ultrasonic images are corrupted by speckle, they contain a variety of features that should be preserved. Existing image filtering methods, however, typically introduce severe blurring in order to adequately suppress the speckle noise, which is composed of low spatial frequencies. In other words, these methods either xe2x80x9csmoothxe2x80x9d the parts of the image one wishes to keep sharp (the useful signal), along with smoothing out the noise, or they fail to eliminate noise that itself is relatively xe2x80x9csmooth.xe2x80x9d
Several methods are known for addressing the problem of speckle. One drawback of most existing speckle-reduction methods is, however, that they are computationally intensive, even for the conventional 2-D imaging for which they are designed. Of course, the problem of computational complexity is much greater in the context of 3-D imaging, in which the number of image brightness values that must be processed will typically be several orders of magnitude greater than in 2-D imaging. This reduces the usefulness of such techniques for high-speed (preferably xe2x80x9creal-timexe2x80x9d) imaging with high resolution and a correspondingly large numbers of pixels.
U.S. Pat. No. 5,594,807 (Liu, Jan. 14, 1997) describes a method and a system for adaptive filtering of ultrasound imaging data in order to remove speckle. In this system, a reference pixel (2-D) is selected, either by the user, or automatically, if possible, from an area of pixels that is assumed to lie in a region of speckle. The system then compiles a reference histogram of the various intensity (xe2x80x9cbrightnessxe2x80x9d) values of the different pixels in the speckle region, grouped into bins. Then, the 2-D scan is performed as normal. The scanned 2-D region itself is partitioned into pixel regions. Brightness histograms are then computed for each pixel region. The histogram of each pixel region is then compared with the reference histogram. The more a pixel region""s histogram matches the reference histogram, the more likely the system assumes it to represent speckle. The contribution (brightness) of pixel regions in the image displayed for the user is then weighted according to the estimated speckle likelihood, with regions of speckle suppressed.
One problem with the system described in U.S. Pat. No. 5,594,807 is that it still does not allow the user to view the 3-D structure of the ultrasonically scanned interrogation region. Returning to the example of a perpendicularly extending blood vessel, the system described in U.S. Pat. No. 5,594,807 would still represent the blood vessel as a ring, although, in most cases, with greater contrast than otherwise.
In the context of 3-D ultrasound imaging, a particularly troublesome consequence of speckle is that it can obscure specular structures of interest. If, for example, a region of speckle lies in the same direction relative to the point of view as, say, a baby""s finger, and if the echo return from the speckle region is stronger than from the finger, then the speckle, that is, the noise, will be displayed rather than the more relevant structure, that is, the finger.
What is needed is an imaging system and method that eliminate or, at least reduce, the effect of speckle in 3-D ultrasound imaging and thereby improve the ability of the system to see body structures that would, in conventional systems, be obscured by speckle. This invention provides such a system and method.
The invention provides a system and a related method for imaging a region of a body. An imaging device, preferably an ultrasound transducer is activated so as to transmit spatially focussed beams of ultrasound or other form of energy into a three-dimensional (3-D) interrogation region (IR) of a body and to receive echo signals from a region of interest (ROI) within the IR. A three-dimensional (3-D) representation of the ROI is then generated as a 3-D pattern of image elements. An intensity value is then assigned to each image element, corresponding to a predetermined imaging property of the image element. Three-dimensional groups of the image elements are then selected at different positions within the ROI and a block property value is computed for each block, the block property values indicating the likelihood that the respective element block corresponds to a region of noise. The intensity values are then projected onto a two-dimensional (2-D) display plane as a function of the block property values and a representation of the display plane is then displayed on a display.
In the preferred embodiment of the invention, a current distribution function of the intensity values of the image elements is evaluated for each of the different blocks. Each block property value is then computed as a predetermined comparison function of the current distribution function and a reference distribution function.
The reference distribution function is preferably determined and evaluated first by selecting a reference region of known noise, preferably a region of known speckle. Reference noise intensity values, which are preferably representations of the intensity of ultrasonic echo return signals from speckle, are then measured from a plurality of points within the reference region. The reference noise intensity values are then grouped into a plurality of intervals that cover an intensity range from a minimum intensity to a maximum intensity. For each interval, a bin value is then assigned that is a count of how many of the reference noise intensity values that lie within the respective interval. The current distribution function is then preferably computed by grouping the intensity values of the image elements in a current one of the element blocks into the plurality of intervals and calculating a predetermined comparison function of the number of current intensity values and reference intensity values in each interval.
One computationally efficient and therefore preferred comparison function is a function of the absolute differences between the number of current intensity values and the number of reference intensity values in each interval. One example of the comparison function that may be used to advantage is a function of the sum of the squares of the absolute differences between the number of current intensity values and the number of reference intensity values in each interval.
In the preferred embodiment of the invention, if the block property value indicates that the element block corresponds to a region of noise, the intensity values of that element block are scaled down in order to reduce the likelihood that they will end up in and degrade the displayed image.
The preferred method for projecting the intensity values onto the 2-D display plane is a maximum intensity projection. (MIP) which entails selecting a point of view; projecting a plurality of vectors from the point of view and through the three-dimensional (3-D) representation of the interrogation region; and selecting for display on the display plane the image element whose intensity value is greatest along each respective vector.
The invention also provides for color-coding the display is order to even further help the user distinguish between noise and structure and to reduce the visible effect of imaged noise. In this embodiment of the invention, a color is associated with each of a plurality of block property value intervals. When the representation of the display plane is displayed, the system renders each image element displayed with the color associated with the block property value interval in which its corresponding block property value falls.